Hello Milosz, On Wed, Sep 24, 2014 at 11:46 PM, Milosz Tanski <milosz@xxxxxxxxx> wrote: > This patcheset introduces an ability to perform a non-blocking read from > regular files in buffered IO mode. This works by only for those filesystems > that have data in the page cache. > > It does this by introducing new syscalls new syscalls preadv2/pwritev2. These > new syscalls behave like the network sendmsg, recvmsg syscalls that accept an > extra flag argument (RWF_NONBLOCK). > > It's a very common patern today (samba, libuv, etc..) use a large threadpool to > perform buffered IO operations. They submit the work form another thread > that performs network IO and epoll or other threads that perform CPU work. This > leads to increased latency for processing, esp. in the case of data that's > already cached in the page cache. > > With the new interface the applications will now be able to fetch the data in > their network / cpu bound thread(s) and only defer to a threadpool if it's not > there. In our own application (VLDB) we've observed a decrease in latency for > "fast" request by avoiding unnecessary queuing and having to swap out current > tasks in IO bound work threads. Since this is a change to the user-space API, could you CC future versions of this patch set to linux-api@xxxxxxxxxxxxxxxx please, as per Documentation/SubmitChecklist. See also https://www.kernel.org/doc/man-pages/linux-api-ml.html. Thanks, Michael > Version 3 highlights: > - Down to 2 syscalls from 4; can user fp or argument position. > - RWF_NONBLOCK value flag is not the same O_NONBLOCK, per Jeff. > > Version 2 highlights: > - Put the flags argument into kiocb (less noise), per. Al Viro > - O_DIRECT checking early in the process, per. Jeff Moyer > - Resolved duplicate (c&p) code in syscall code, per. Jeff > - Included perf data in thread cover letter, per. Jeff > - Created a new flag (not O_NONBLOCK) for readv2, perf Jeff > > > Some perf data generated using fio comparing the posix aio engine to a version > of the posix AIO engine that attempts to performs "fast" reads before > submitting the operations to the queue. This workflow is on ext4 partition on > raid0 (test / build-rig.) Simulating our database access patern workload using > 16kb read accesses. Our database uses a home-spun posix aio like queue (samba > does the same thing.) > > f1: ~73% rand read over mostly cached data (zipf med-size dataset) > f2: ~18% rand read over mostly un-cached data (uniform large-dataset) > f3: ~9% seq-read over large dataset > > before: > > f1: > bw (KB /s): min= 11, max= 9088, per=0.56%, avg=969.54, stdev=827.99 > lat (msec) : 50=0.01%, 100=1.06%, 250=5.88%, 500=4.08%, 750=12.48% > lat (msec) : 1000=17.27%, 2000=49.86%, >=2000=9.42% > f2: > bw (KB /s): min= 2, max= 1882, per=0.16%, avg=273.28, stdev=220.26 > lat (msec) : 250=5.65%, 500=3.31%, 750=15.64%, 1000=24.59%, 2000=46.56% > lat (msec) : >=2000=4.33% > f3: > bw (KB /s): min= 0, max=265568, per=99.95%, avg=174575.10, > stdev=34526.89 > lat (usec) : 2=0.01%, 4=0.01%, 10=0.02%, 20=0.27%, 50=10.82% > lat (usec) : 100=50.34%, 250=5.05%, 500=7.12%, 750=6.60%, 1000=4.55% > lat (msec) : 2=8.73%, 4=3.49%, 10=1.83%, 20=0.89%, 50=0.22% > lat (msec) : 100=0.05%, 250=0.02%, 500=0.01% > total: > READ: io=102365MB, aggrb=174669KB/s, minb=240KB/s, maxb=173599KB/s, > mint=600001msec, maxt=600113msec > > after (with fast read using preadv2 before submit): > > f1: > bw (KB /s): min= 3, max=14897, per=1.28%, avg=2276.69, stdev=2930.39 > lat (usec) : 2=70.63%, 4=0.01% > lat (msec) : 250=0.20%, 500=2.26%, 750=1.18%, 2000=0.22%, >=2000=25.53% > f2: > bw (KB /s): min= 2, max= 2362, per=0.14%, avg=249.83, stdev=222.00 > lat (msec) : 250=6.35%, 500=1.78%, 750=9.29%, 1000=20.49%, 2000=52.18% > lat (msec) : >=2000=9.99% > f3: > bw (KB /s): min= 1, max=245448, per=100.00%, avg=177366.50, > stdev=35995.60 > lat (usec) : 2=64.04%, 4=0.01%, 10=0.01%, 20=0.06%, 50=0.43% > lat (usec) : 100=0.20%, 250=1.27%, 500=2.93%, 750=3.93%, 1000=7.35% > lat (msec) : 2=14.27%, 4=2.88%, 10=1.54%, 20=0.81%, 50=0.22% > lat (msec) : 100=0.05%, 250=0.02% > total: > READ: io=103941MB, aggrb=177339KB/s, minb=213KB/s, maxb=176375KB/s, > mint=600020msec, maxt=600178msec > > Interpreting the results you can see total bandwidth stays the same but overall > request latency is decreased in f1 (random, mostly cached) and f3 (sequential) > workloads. There is a slight bump in latency for since it's random data that's > unlikely to be cached but we're always trying "fast read". > > In our application we have starting keeping track of "fast read" hits/misses > and for files / requests that have a lot hit ratio we don't do "fast reads" > mostly getting rid of extra latency in the uncached cases. > > I've performed other benchmarks and I have no observed any perf regressions in > any of the normal (old) code paths. > > > I have co-developed these changes with Christoph Hellwig. > > Milosz Tanski (4): > vfs: Prepare for adding a new preadv/pwritev with user flags. > vfs: Define new syscalls preadv2,pwritev2 > vfs: Export new vector IO syscalls (with flags) to userland > vfs: RWF_NONBLOCK flag for preadv2 > > arch/x86/syscalls/syscall_32.tbl | 2 + > arch/x86/syscalls/syscall_64.tbl | 2 + > drivers/target/target_core_file.c | 6 +- > fs/cifs/file.c | 6 ++ > fs/nfsd/vfs.c | 4 +- > fs/ocfs2/file.c | 6 ++ > fs/pipe.c | 3 +- > fs/read_write.c | 121 +++++++++++++++++++++++++++++--------- > fs/splice.c | 2 +- > fs/xfs/xfs_file.c | 4 ++ > include/linux/aio.h | 2 + > include/linux/fs.h | 7 ++- > include/linux/syscalls.h | 6 ++ > include/uapi/asm-generic/unistd.h | 6 +- > mm/filemap.c | 22 ++++++- > mm/shmem.c | 4 ++ > 16 files changed, 163 insertions(+), 40 deletions(-) > > -- > 2.1.0 > > -- > To unsubscribe, send a message with 'unsubscribe linux-aio' in > the body to majordomo@xxxxxxxxx. For more info on Linux AIO, > see: http://www.kvack.org/aio/ > Don't email: <a href=mailto:"aart@xxxxxxxxx";>aart@xxxxxxxxx</a> -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Author of "The Linux Programming Interface", http://blog.man7.org/ -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
